Conventionally, cellulose acylate films have been stretched to generate in-plane retardation (Re) and retardation in the thickness direction (Rth), and it has been practiced to extend viewing angles by using such film as a retardation film in a liquid crystal display device.
Examples of methods of stretching a cellulose acylate film include a method of stretching in the longitudinal (length) direction (longitudinal stretching), a method of stretching in the transverse (width) direction (transverse stretching), and a method of carrying out longitudinal stretching and transverse stretching simultaneously (simultaneous biaxial stretching). Of these, because of the compactness of the equipment, longitudinal stretching has been employed in many cases so far. Generally, in the longitudinal stretching, a film is heated to the glass transition temperature (Tg) or higher on at least two pairs of nip rolls, and stretched in the longitudinal direction with setting the carrying rate of the nip roll on the exit side faster than that of the nip roll on the entrance side.
Patent Document 1 describes a method of longitudinal stretching of cellulose ester. In Patent Document 1, by carrying out longitudinal stretching in the direction opposite to the direction of casting film, angle irregularities of the lagging axis is improved. Patent Document 2 describes a method of stretching in which nip rolls positioned with a small span of a length/width ratio (L/W) of 0.3 or more and 2 or less are provided in the stretching zone. According to Patent Document 2, the orientation in the thickness direction (Rth) can be improved. The length/width ratio described herein means a value obtained by dividing the distance (L) between the nip rolls used for stretching by the width (W) of a cellulose acylate film to be stretched.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2002-311240
[Patent Document 2] Japanese Patent Application Laid-Open No. 2003-315551
However, when stretched cellulose acylate films produced by the methods described in Patent Documents 1 and 2 are used as a retardation film for a liquid crystal display device, minute display irregularities may be generated, and the films are not of satisfactory quality as a retardation film for a liquid crystal display device.
Conventional cellulose acylate films produced by melt film forming have a defect that they tend to be broken upon stretching when the stretching ratio is increased, and when they are used as a retardation film, the stretching ratio cannot be increased to the level sufficient for achieving the desired in-plane retardation (Re) and retardation in the thickness direction (Rth), and therefore a highly oriented stretched cellulose acylate film cannot be obtained.
The present invention has been made in view of such circumstances and aims at providing a cellulose acylate film from which a highly oriented stretched cellulose acylate film can be obtained by producing an unstretched cellulose acylate film hardly breakable upon stretching by melt film forming, a method of producing the same, a stretched cellulose acylate film and a method of producing the same.